Methods of producing a polyether-polyamide block copolymer containing a polyoxyalkylene polymer moiety and a polylactam polymer moiety by means of anionic polymerization in the presence of an anionic polymerization catalyst are described in, for example, Japanese patent publication No. 40,120/79 and U.S. Pat. Nos. 3,862,262, 4,031,164, 4,034,015 and 4,223,112; these patents indicate that said block copolymer can be used for textile fibers, foams, furnitures and automotive parts because of its excellent properties. However, one of the main disadvantages of the polyether-polyamide block copolymer obtained by the method of the patents shown above is that the copolymer is colored yellow, and because of this defect, colorless products cannot be obtained, and even when it is desirable to give certain colors to products, only dull colors are obtained.
German patent application (OLS) No. 1,909,577 describes that anionic polymerization of a lactam can be effected using as an activator an ester of a lactam-N-carboxylic acid and a di- or polyol compound represented by the formula: ##STR2## wherein R is an aliphatic group having 2 to 12 carbon atoms, and m is an integer of 4 to 12, and X and Y are each a hydrogen atom or a residual group of ##STR3##
However, the object of said patent is to produce a polyamide by homopolymerization of a lactam, but not to produce a block copolymer of a polyoxyalkylene and a polyamide, as in the case of the present invention.
In order to overcome the defects of the prior art methods, the inventors have conducted intensive research on a method of obtaining a colorless polyether-polyamide block copolymer of excellent qualities, and have finally achieved the present invention.